This invention relates to an apparatus for measuring the pre-tension of a threaded bolt used, for example, for closing a reactor pressure vessel; such a bolt is elongated by means of a tensioning device that is portable and can easily be put on and off. The tensioning device may contain a tensioning bolt. The apparatus includes a two part measuring rod which is introduced into a central, closed hole in the threaded bolt and extends down to the lowest point of the hole. The rod passes through the tensioning device and also through the tensioning bolt, if one is used. A length measuring device which determines the difference in length between the threaded bolt and the measuring rod is included.
One such measuring apparatus is shown in U.S. Pat. No. 3,877,326. There, both the threaded bolt used for closing a reactor pressure vessel and the automatic tensioning device which could be put on for supplying the pretension with, optionally, a tensioning bolt, contain contiguous central bore holes forming a passage which is closed at the lowest point of the threaded bolt. A two part measuring rod is introduced into the hole and is screwed in at the lowest point of the hole. When the threaded bolt is elongated and, thus, pretensioned by means of the tensioning device, the measuring rod located in the hole remains unstressed and the pre-tension of the threaded bolt can be determined by measuring the difference in length between the inserted measuring rod and the threaded bolt including the tensioning bolt, if one is used; this difference is determined by a length measuring device such as a dial indicator.
In the measuring device just described, the measuring rod used for measuring the pre-tension must be inserted by hand into the central passage of the threaded bolt as well as that of any tensioning bolt, an operation which takes a substantial amount of time, especially since these parts must be removed again after the measurement. It is an object of the present invention to provide a measuring apparatus which is fastened to the tensioning device, can be transported with it, and in which the measuring rod can be actuated automatically.